A phalanx in the metatarsus: the human first ray preserves ancestral digital identity

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Camasta This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9283272/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract The human first metatarsal (MT1) has long been regarded as a true metapodial uniquely adapted for bipedal propulsion. Yet developmental, vascular, and morphometric evidence increasingly contradicts this view. Across 100 adult human foot specimens, MT1 shares its proportions, nutrient-artery entry, and physeal orientation with proximal phalanges rather than with metatarsals II–V. Developmentally, MT1 arises after the other metatarsals but before the phalanges, defining a proximal-phalangeal timing sequence. Its joint and ligamentous architecture reveals isolation from the Lisfranc complex, while its gene-expression domain lies within the distal Hox field characterized by Hoxd13-dominant regulation. Collectively, these features identify MT1 as a proximally displaced phalanx, with the os intermetatarseum representing a suppressed ancestral metapodial. This reinterpretation reframes the human foot as a pentadactyl yet quadrametapodial limb, integrating anatomy, development, and evolution within a preserved-polarity model grounded in fossil and comparative anatomical evidence. Health sciences/Anatomy Biological sciences/Developmental biology Biological sciences/Evolution Biological sciences/Zoology Figures Figure 1 Figure 4 INTRODUCTION Classical anatomists recognized that the first ray—the hallux with its supporting bones (MT1, proximal and distal phalanges)—differs markedly from the lesser rays (MT2–5 with their proximal, middle, and distal phalanges) in shape and mobility. Aristotle¹ and Galen² described the big toe as a distinct “thumb of the foot,” and Vesalius³ noted that it possessed only two phalanges, unlike the remaining digits that have three. From a functional and evolutionary point of view, the human foot has been characterized by derived features associated with habitual bipedalism, including (i) loss of plantar grip; (ii) loss of prehensile capacity; (iii) loss of mobility of the first ray, which has become permanently adducted against the second; and (iv) medial translation of the functional axis from the primitive third-ray location (mesaxony) to a second-ray configuration (entaxony).⁴ Morton synthesized these observations into a functional framework in which the first ray is a specialized propulsive lever within the medial column,⁵ while Wood Jones similarly emphasized the transformation of the hallux from a prehensile structure into a rigid, adducted element aligned with the medial longitudinal arch.⁶ Recent morphometric, embryological, and genetic evidence challenges the assumption that MT1 is merely a modified metapodial, suggesting instead that the first ray may represent a phalanx-derived digit incorporated proximally within the tarsometatarsal column. The present study integrates osteometric, vascular, ligamentous, developmental, and genetic evidence to test the hypothesis that the first metatarsal is not a metapodial by identity, but a proximally displaced proximal phalanx. OSTEOLOGY AND MORPHOMETRICS To evaluate the structural identity (rather than positional classification) of the human first ray, osteometric analyses were performed on 100 adult dry-bone foot specimens. The first ray was analyzed as a three-segment unit (MT1 + PP1 + DP1) and compared with the second toe (PP2 + MP2 + DP2), which represents a canonical three-phalangeal digital series. This analysis tests whether the proximal element of the first ray conforms to a metapodial or a proximal-phalangeal morphometric program. ⁷ Maximum proximodistal length and mediolateral diaphyseal width were measured for metatarsals I–V and for the proximal phalanx of digit II (PP2). Length-to-width (L/W) ratios were calculated to characterize relative scale independent of size (Table 1 ) . Table 1 Length-to-width (L/W) ratios of metatarsals I–V and proximal phalanx II (PP2) Element Mean L/W SE 95% CI p (vs MT1) CV (%) MT1 4.83ᵃ 0.048 4.73–4.92 — 9.9 MT2 10.00ᶜ 0.116 9.77–10.23 < 0.0001 11.6 MT3 10.34ᵈ 0.104 10.13–10.54 < 0.0001 10.1 MT4 10.02ᶜ 0.101 9.82–10.22 < 0.0001 10.1 MT5 8.65ᵇ 0.128 8.39–8.90 < 0.0001 14.8 PP2 4.68ᵃ 0.078 4.53–4.84 0.73 10.2 Mean length-to-width (L/W) ratios are shown for metatarsals I–V (MT1–MT5) and the proximal phalanx of digit II (PP2) across 100 adult human feet. Values are reported as mean ± standard error (SE), with 95% confidence intervals. Group differences were assessed using a randomized-block ANOVA (feet as blocks; skeletal elements as treatments), followed by Tukey’s honestly significant difference (HSD) post-hoc test. Elements sharing the same superscript letter do not differ significantly at α = 0.05. MT1 and PP2 form a statistically indistinguishable group, distinct from metatarsals II–V, supporting serial homology of the first ray proximal element with a proximal phalanx. Whole-ray architecture was assessed by normalizing segment lengths to the middle segment (PP1 for the first ray; MP2 for the second toe), allowing direct comparison of internal digital proportions (Fig. 1 ). Scaling relationships between length and width were examined to determine whether MT1 aligns with metapodial or phalangeal morphospace (Fig. 2 ), and normalized segmental proportions are summarized in Fig. 3 . RESULTS Morphometric analyses demonstrate that the first metatarsal occupies a morphospace distinct from the metatarsal series and congruent with proximal phalanges. The mean length-to-width (L/W) ratio of MT1 (4.83 ± 0.48) was less than half that of metatarsals II–V (8.65–10.34; all p < 0.0001), while MT1 and the proximal phalanx of digit II (PP2) did not differ significantly (p = 0.73; Table 1 ). Patterns of variability further distinguish MT1 from the metatarsal series. The length/width ratios for MT1 closely matched that of PP2 and these were substantially lower than those of metatarsals II–V. This indicates shared developmental constraint within a phalangeal module rather than the broader variability characteristic of metapodial elements. Coefficient of variance being relatively equal for all elements implies a common developmental constraint. Whole-ray proportional analysis revealed that the first ray preserves a canonical three-segment digital architecture. After normalization to the middle segment, proximal and distal segment ratios of the first ray clustered near 2.1 and 0.79, respectively, and were indistinguishable from those of the second toe (Fig. 3 ). These proportional relationships were maintained across the full range of absolute sizes observed, demonstrating conservation of internal digital scaling despite variation in overall ray length. DISCUSSION These morphometric data demonstrate that the first metatarsal does conform to a phalanx-like element incorporated proximally within the first ray. MT1 matches proximal phalanges in absolute proportions, variance, and length–width scaling, while remaining clearly distinct from the canonical metatarsal program governing rays II–IV. Whole-ray proportional analysis is particularly revealing. Despite its proximal position, the first ray preserves a three-segment digital architecture indistinguishable from that of the second toe. This pattern is incompatible with interpretations of MT1 as a hypertrophied or remodeled metapodial, but follows directly if MT1 represents a proximally displaced proximal phalanx retaining its ancestral scaling relationships. Together with its low L/W ratio, and preservation of canonical three-segment digital proportions, these findings indicate that MT1 is a morphometrically constrained, phalanx-like element rather than a modified metapodial. As shown in the sections that follow, this morphometric signal is not coincidental but recurs across independent anatomical, developmental, vascular, and evolutionary observations that together bear on the identity of the first ray. LIGAMENTOUS AND JOINT ARCHITECTURE The first tarsometatarsal joint (TMT1) is anatomically and synovially isolated from the lesser tarsometatarsal complex. Unlike TMT2–TMT5, which are united within a continuous synovial compartment—the classical “great tarsal synovial cavity”—TMT1 possesses a discrete, independent joint capsule with no synovial communication to adjacent tarsometatarsal joints. ⁸ − ¹⁰ Metatarsals II–V are bound together by confluent dorsal, plantar, and interosseous ligaments, forming a rigid intermetapodial unit reinforced by the Lisfranc ligament, which spans from the medial cuneiform to the base of MT2. TMT1 is excluded from this complex entirely: it lacks interosseous connections to the lesser metatarsals, is bypassed by the Lisfranc ligament, and articulates through an autonomous capsular system. This synovial segregation is not a minor variation in mobility but a categorical distinction in joint organization. Whereas TMT2–TMT5 function as a unified metapodial block, TMT1 behaves as an independent articulation, structurally and developmentally analogous to a metatarsophalangeal joint rather than a true intermetapodial junction. The isolated capsule and absence of shared synovial space reinforce the interpretation that the first ray is excluded from the Lisfranc complex, consistent with preserved phalangeal identity rather than a modified metapodial element. VASCULAR AND PHYSEAL PARALLELS MT1 exhibits a vascular and physeal pattern parallel to the phalanges rather than the metatarsals. Its nutrient artery enters proximally on the plantar base, a configuration shared by proximal phalanges but not by metatarsals II–V, whose nutrient foramina are typically mid-diaphyseal.¹¹ , ¹² Likewise, MT1 possesses a proximal epiphysis, matching phalangeal polarity rather than the distal epiphyseal orientation of the lesser metatarsals.¹³ Together, these features indicate that MT1 follows a phalangeal—not metapodial—developmental program. EMBRYONIC FOOT DEVELOPMENT AND THE OS INTERMETATARSEUM Embryonic skeletal elements of the human foot develop in a skewed, non-metameric order of chondrification that places the first ray out of sequence with the lesser metatarsals. Metatarsals II–V chondrify first, followed by delayed appearance of MT1 and then the proximal phalanges of digits II–V. Within the hallux, PP1 forms just prior to the middle phalanges of digits II–V, while DP1 develops in parallel with DP2–DP5.¹⁴ , ¹⁵ This sequence aligns the first ray developmentally with the digital program of rays II–V rather than with the metatarsal cohort. During this same interval, a constant transient primordium appears between the bases of MT1 and MT2 within the embryonic metatarsus, corresponding to the os intermetatarseum (OI).¹⁵ , ¹⁶ At the 25 mm crown–rump length this primordium may chondrify, but it quickly dedifferentiates and rapidly disappears after reaching the 30 mm crown–rump length.¹⁵ At maximal development, it attains a quarter of the length of the first intermetatarsal space and “bears the character of a structure recapitulated in the course of ontogenesis.”¹⁵ In adults, the OI may persist as an accessory ossicle or fuse variably to the base of either MT1 or MT2.¹⁷ − ¹⁹ The identity of this element has been debated for more than a century. Gruber interpreted it as “split-off material of an originally bulkier and longer tarsale distale I,”¹⁶ while Pfitzner proposed that it represented a persisting ancestral metatarsal or tarsal element.²⁰ Čihák rejected this interpretation, noting that “in paleontology there is known neither a case of six elements in the distal tarsal row nor an increased number in Theromorphs,” which “are all five-toed.”¹⁵ Slabý later emphasized that “phylogenetic evidence can scarcely be found for the theory which considers this bone a persisting ancestral metatarsal element.”²¹ Arising at the 16–30 mm crown–rump length, the OI appears before the delayed chondrification of MT1 and in parallel with MT2–MT5, placing this vestigial element within the ancestral metapodial domain and underscoring MT1’s displacement from the typical metatarsal program toward a phalangeal identity.¹⁰ , ¹⁴ − ¹⁶ Postnatal skeletal maturation mirrors this embryonic distinction. The proximal physis of MT1 ossifies completely at approximately 15–17 years, coincident with closure of the phalangeal physes, whereas the distal growth plates of metatarsals II–V typically remain open until late adolescence (~ 17–20 years). ¹⁰ , ¹³ The earlier cessation of growth in MT1 thus conforms to a phalangeal rather than metatarsal maturation schedule. HAND HOMOLOGY Independent evidence from the human hand supports the interpretation that the first ray preserves phalangeal identity through proximal displacement. Radiographic and developmental analyses have demonstrated that the first metacarpal (MC1) shares proportional geometry, ossification behavior, and growth dynamics with the proximal phalanges of digits II–V rather than with the remaining metacarpals. Pazzaglia and colleagues concluded that the missing thumb segment is best accounted for by phalangeal homology of MC1 using proportional length assessment, ossification-center distribution, and differential growth rates,²² and Valenzuela and colleagues similarly identified MC1 as homologous to the proximal phalanx based on relative length relationships and delayed maturation of the preaxial digit.²³ Further support comes from analyses of rare five-fingered hand (5-FH) phenotypes in humans, a dominantly inherited condition linked to chromosome 7q36.²⁴ In these individuals, Bondioni et al. demonstrated that MC1 exhibits postnatal growth asymmetry and proportional scaling indistinguishable from proximal phalanges and distinct from metacarpals II–V, supporting phalangeal homology of the thumb’s proximal element.²⁵ Developmental studies of the growing hand likewise show delayed ossification and maturation of the preaxial (radial/anterior) digit relative to the postaxial digits, both prenatally and postnatally, mirroring developmental polarity patterns observed in the foot.²³ The concordant pattern in hand and foot reinforces the interpretation that the first ray retains intrinsic phalangeal identity despite occupying a proximal position. FUNCTIONAL AND CLINICAL IMPLICATIONS Why is MT1 so thick and broad compared with metatarsals II–V? Multiple pedobarographic studies demonstrate that MT1 does not bear the greatest plantar pressure during gait,²⁶ , ²⁷ with peak pressures instead concentrated beneath the hallux and central forefoot rather than the first metatarsal head.²⁸ Reliability of the measurement for plantar pressure parameters is high.²⁹ The transverse robustness of MT1 therefore does not reflect increased peak load bearing, but structural resistance and the mechanical demands of the sesamoid–pulley apparatus. Its shape is optimized for leverage during terminal stance, consistent with reinforcement of a proximal-phalangeal element rather than hypertrophy of a metapodial.³⁰ The metatarsal parabola (MT2 > MT3 > MT4 > MT5) is highly conserved across individuals. By contrast, MT1 exhibits greater variability in projection length relative to MT2 across both osteometric and radiographic analyses.³¹ − ³⁴ Some feet display a relatively longer MT1, whereas others exhibit a shorter one; nevertheless, this variability does not alter the internal proportional scaling of the first ray relative to the second toe. Pathologic extremes of first-ray macrodactyly and microdactyly demonstrate preservation of proximal-to-distal segment ratios despite marked size differences (Fig. 4 ). COMPARATIVE AND EVOLUTIONARY CONTEXT Comparative and fossil evidence show that the two-phalangeal condition of digit I is among the most conserved features of the tetrapod limb. Early tetrapods rapidly stabilized a pentadactyl pattern in which digit I possessed fewer phalanges than the central digits. By the time of early amniotes, a digital formula approximating 2-3-4-5-3 was established, and subsequent synapsid and mammalian evolution consolidated this pattern into the canonical mammalian formula 2-3-3-3-3. This extraordinary persistence indicates that digit I occupies a uniquely constrained developmental domain within the autopod. Whereas digits II–V underwent progressive reduction and reorganization during amniote diversification, digit I consistently retained a short, two-phalangeal architecture. Such long-term stability favors preservation of distal digital identity rather than repeated modification of a metapodial element. In primates, the first ray maintained pronounced medial mobility and a grasping role. Fossil hominins document its subsequent transformation into a load-bearing component of the human foot. In Ardipithecus ramidus (~ 4.4 Ma), the first ray remained abducted and opposable, yet already exhibited transverse robustness within the modern human range, indicating structural reinforcement prior to the evolution of rigid longitudinal and transverse arches.³⁵ Through Australopithecus afarensis and early Homo , the first ray progressively adducted toward the second ray, integrating into the developing medial column of the foot.³⁶ − ⁴³ The modern human foot represents reorganization rather than a departure from ancestral patterning. Despite profound functional change—from grasping to propulsion—the intrinsic digital formula of the first ray was preserved. Evolutionary innovation proceeded through spatial redeployment of conserved developmental elements, not through reversal of polarity or loss of digital identity (Table 2 ). Table 2 Continuity of the two-phalangeal first digit across vertebrate evolution. Taxon / Group Approx. Age (Ma) Phalangeal Formula (I–V) Key Features Polydactyl stem-tetrapods ~ 365–360 > 5 digits Transitional limbs with variable digit counts First pentadactyl tetrapods ~ 360–350 2-3-4-5-3 Fixed five rays; digit I already two phalanges Basal amniotes ~ 320–310 2-3-4-5-3 Conserved two-phalangeal digit I Early synapsids ~ 305–275 2-3-4-4-3 Digit I stable; central digits reduced Basal mammals ~ 200–160 2-3-3-3-3 Mammalian formula fixed Primates ~ 66–50 2-3-3-3-3 Opposable hallux; grasping feet Ardipithecus ramidus ~ 4.4 2-3-3-3-3 Abducted hallux; elongate and robust first ray Australopithecus afarensis ~ 3.5 2-3-3-3-3 Inline hallux; incipient arch Homo sapiens 0 2-3-3-3-3 Full arches; rigid first ray Chronological summary of phalangeal formulas (digits I–V) from early tetrapods to Homo sapiens, illustrating the long-term conservation of the two-phalangeal condition of digit I. Progressive reduction and consolidation of central digits produced the canonical mammalian pattern 2-3-3-3-3, retained through primates and hominins and foundational to the preserved-polarity model of the human first ray. HOX GENE REGULATION AND PRESERVED POLARITY Patterning of the vertebrate limb is governed by the Hox gene family, a conserved group of transcription factors that establish positional identity along the proximal–distal axis of the appendicular skeleton. This axis is classically divided into the stylopod (proximal segment), zeugopod (intermediate segment), and autopod (distal segment comprising the hands and feet). Within the autopod, distinct combinations of Hox gene expression specify metapodial versus phalangeal identity and regulate digital segmentation. ⁴⁴ − ⁵⁰ Digit I occupies a uniquely truncated developmental domain within this framework. Unlike digits II–V, which express overlapping domains of Hoxd11, Hoxd12, and Hoxd13, digit I is specified almost exclusively by Hoxd13, with little or no contribution from the more proximal paralogues. This restricted expression domain establishes intrinsic phalangeal identity and limits segmentation, accounting for the conserved two-phalangeal structure of digit I across tetrapods.⁴⁴ − ⁴⁸ Classical reverse-polarity proponents interpret the first metatarsal as a true metapodial that secondarily acquired phalangeal characteristics through inversion of developmental polarity.⁵¹ Under this model, MT1 is presumed to retain metapodial identity despite its phalange-like morphology, with distal regulatory programs imposed upon an originally proximal skeletal element. In the human first ray, however, the distal identity program is not reversed but spatially displaced. MT1 resides within a Hoxd13-dominant regulatory field rather than within the mixed Hox domains characteristic of metatarsals II–V. This placement is consistent with MT1’s phalangeal morphometrics, proximal vascular entry, proximal physeal polarity, delayed chondrification, and ligamentous isolation. Polarity is therefore preserved rather than inverted: the ancestral first metapodial program was developmentally suppressed—its vestige appearing as the os intermetatarseum—while phalangeal identity expanded proximally into the tarsometatarsal column. Experimental manipulation of Hox13 paralogues supports this interpretation. Loss of Hox13 function disrupts phalangeal segmentation, whereas ectopic activation of Hoxd13 transforms metapodial condensations into phalangeal-like elements. Conversely, deletion of distal regulatory enhancers collapses multiple phalanges into a single condensation. Together, these findings demonstrate that Hox13 paralogues act as terminal selector genes for phalangeal identity. In the first ray, this identity module has been redeployed proximally without alteration of its intrinsic developmental logic. The preserved-polarity model thus resolves the apparent paradox of the human first ray. Rather than representing a metapodial that secondarily acquired phalangeal features, MT1 is a phalangeal element by identity, incorporated proximally through spatial reorganization of conserved developmental programs. This interpretation integrates genetic regulation with embryologic timing, comparative anatomy, and morphometric constraint, and provides a mechanistic foundation for understanding the evolutionary transformation of the human foot. REASSESSMENT OF THE REVERSE-POLARITY MODEL Reno and colleagues proposed that the first metatarsal represents a true metapodial that secondarily acquired phalangeal features through reversal of developmental polarity. ⁵¹ Under this reverse-polarity model, MT1 is interpreted as homologous with metatarsals II–V despite its phalange-like proportions, proximal epiphysis, and atypical vascular and ligamentous anatomy. The defining criterion for metapodial identity in this framework is positional: articulation with a distal tarsal is taken to supersede developmental, morphometric, and genetic evidence. When evaluated using the evidence presented here, this model is not parsimoniously supported. First, positional articulation alone does not define serial homology. The os intermetatarseum occupies the predicted locus of the ancestral first metapodial between the medial cuneiform and MT2, indicating that the original metapodial condensation was developmentally suppressed rather than transformed. Recognition of this element resolves the positional paradox without invoking polarity reversal. Second, the anatomical features cited as evidence for polarity reversal form a coherent phalangeal signature rather than a labile metapodial variant. MT1 consistently exhibits phalange-like morphometrics, low proportional variance, proximal nutrient-artery entry, proximal physeal polarity, delayed chondrification, and isolation from the Lisfranc complex. These traits co-occur across specimens and developmental stages and are shared with proximal phalanges but not with metatarsals II–V. Concidering their concordance as independent reversals of multiple developmental axes is not parsimonious. Third, the reverse-polarity model lacks a mechanistic genetic foundation. Hox-gene regulation predicts that elements specified within a Hoxd13-dominant domain will express phalangeal identity, irrespective of their spatial position. MT1 resides within this distal regulatory field, whereas metatarsals II–V do not. Experimental manipulation of Hox13 paralogues demonstrates that phalangeal identity cannot be imposed upon a metapodial framework without altering its developmental program. The observed phenotype of MT1 therefore reflects preserved distal identity rather than inverted polarity. Finally, the reverse-polarity hypothesis implies repeated, independent loss of a true first metapodial across tetrapod evolution. In contrast, the preserved-polarity model requires only a single ancestral suppression event, with the os intermetatarseum persisting as a vestigial remnant. This explanation is more parsimonious and aligns with the long-term conservation of first ray phalangeal identity documented across more than 350 million years of vertebrate evolution. Taken together, the reverse-polarity interpretation fails to accommodate the combined osteometric, developmental, vascular, ligamentous, genetic, and comparative evolutionary evidence. The preserved-polarity model accounts for all observed features within a single, mechanistically grounded framework: MT1 is a proximal phalanx by identity, incorporated proximally through spatial reorganization of conserved developmental programs rather than through reversal of limb polarity. Reno et al. correctly emphasize that TMT1 is ligamentously and capsularly segregated from the Lisfranc complex, but their proposed mechanism is inverted. They argue that a proximal growth plate “precluded” intermetatarsal union in the first ray, with compensatory joint-shape changes. Under the preserved-polarity model, however, the absence of intermetatarsal ligaments is not a deficiency produced by an unusual physis; it is the expected anatomy of a non-metapodial element occupying the first-ray position. In the lateral rays, metapodial identity is expressed as a shared synovial–ligamentous complex (the Lisfranc unit) that couples MT2–MT5 into a stabilized platform with minimal triplanar excursion. By contrast, TMT1 behaves as an isolated, triplanar articulation precisely because its proximal element is phalangeal by identity: it lacks the intermetapodial “zippering” architecture that defines the metatarsal series. This interpretation simultaneously accounts for (i) the absence of Lisfranc linkage, (ii) proximal physeal polarity, (iii) proximal nutrient-artery entry, (iv) delayed chondrification sequence, and (v) phalangeal scaling, without invoking a compensatory narrative or independent polarity reversals. Taken together, the ligamentous, vascular, developmental, morphometric, and genetic evidence converges on element identity rather than positional reinterpretation. These findings fulfill Remane’s criteria for homology—special quality, topological relations, and continuity through intermediates—demonstrating that MT1 reproduces phalangeal geometry and developmental polarity while maintaining continuity through the os intermetatarseum and fossil intermediates. 52 DISCUSSION AND SYNTHESIS The human first ray exemplifies how profound functional transformation can arise through developmental conservation rather than developmental reversal. In ancestral primates, the hallux was shortened, opposable, and constrained within a phalangeal identity suited for grasping. With the advent of habitual bipedalism, the first ray was progressively lengthened, adducted, and reinforced to participate in the medial longitudinal and transverse arches of the foot. Fossil intermediates demonstrate that transverse robustness of the first ray preceded full arch rigidity, indicating structural strengthening before the complete loss of prehensility. Crucially, this transformation did not require reversal of developmental polarity. Instead, a phalanx-derived element was incorporated proximally into the tarsometatarsal column, preserving intrinsic digital identity while accommodating new mechanical demands. The os intermetatarseum represents the vestigial remnant of a suppressed ancestral metapodial, revealing that the tetrapod limb is fundamentally pentadactyl and triphalangeal, yet quadrametapodial in its developmental architecture. The modern human first ray thus represents a robust, phalanx-derived element whose proportions and scaling preserve those of a typical three-segment digit, repurposed for upright locomotion. Bipedal propulsion was achieved not by abandoning ancestral patterning, but by spatial reorganization of conserved developmental modules. In this light, the apparent uniqueness of the human first ray reflects not evolutionary novelty, but the long-term stability and flexibility of a deeply conserved developmental framework. In this context, the first ray illustrates a broader evolutionary tendency toward the reuse and reorganization of established anatomical elements rather than the generation of novel ones. Declarations Methods – Statistical Analysis Morphometric analyses were conducted using Python (v3.11) with the SciPy and StatsModels libraries. For each specimen, proximodistal length and mediolateral diaphyseal width were measured for metatarsals I–V (MT1–MT5) and for the proximal phalanx of digit II (PP2). Length-to-width (L/W) ratios were calculated to quantify relative robustness independent of absolute size. To account for repeated measures within individual feet, group-level differences in L/W ratios were evaluated using a randomized-block analysis of variance (ANOVA), with feet treated as blocks and skeletal elements treated as fixed effects. Post-hoc pairwise comparisons were performed using Tukey’s Honestly Significant Difference (HSD) test. Group means are reported with standard errors (SE) and 95% confidence intervals. CV = (s/ȳ) x 100. All methods were carried out in accordance with relevant guidelines and regulations. All raw data and variable definitions are publicly available, enabling independent verification and replication of all reported analyses. Data Availability All raw morphometric data generated and analyzed during this study are publicly available in the Dryad Digital Repository (https://doi.org/10.5061/dryad.9s4mw6mxp). Code Availability No custom code was used in this study. All analyses were performed using standard statistical methods as described in the Methods section. Ethics Statement All methods were carried out in accordance with relevant guidelines and regulations. The material analyzed in this study consisted of de-identified adult human dry-bone foot specimens from the Department of Anatomy at the Pennsylvania College of Podiatric Medicine (now Temple University School of Podiatric Medicine), examined as part of an institutional anatomical teaching and research collection. No living subjects, patient data, or identifiable human materials were involved. Institutional ethical approval and the requirement for informed consent were waived by the Institutional Review Board of Temple University School of Podiatric Medicine. Competing Interests The author declares no competing interests. Funding There is no funding for this project from any outside sources. Author’s Information Craig A. 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The first metacarpal is the thumb first phalanx: Evo-Devo implications. Int. J. Morphol. 27 , 985–988 (2009). Dobbs, M. B. et al. Triphalangeal thumb locus maps to chromosome 7q36. J. Orthop. Res. 18 , 340–344 (2000). Bondioni, M. P. et al. Normal and five-fingered hand: comparative X-ray morphometry. Folia Morphol. 80 , 403–409 (2021). Cavanagh, P. R., Rodgers, M. M. & Iiboshi, A. Pressure distribution under symptom-free feet during barefoot standing. Foot Ankle . 7 , 262–276 (1987). Hutton, W. C. et al. Forces acting on the metatarsals during walking. J. Anat. 132 , 287–294 (1981). Buldt, A. K. et al. Foot posture is associated with plantar pressure during gait. Clin. Biomech. 28 , 123–130 (2013). Cousins, S. D., Morrison, S. C. & Drechsler, W. I. The reliability of plantar pressure assessment during barefoot level walking in adults: a systematic review. Gait Posture . 39 , 784–790 (2014). Hicks, J. H. The mechanics of the foot. II. The plantar aponeurosis and the arch. J. 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Functional morphology of the ankle and the likelihood of climbing in early hominins. Proc. Natl Acad. Sci. USA 106, 6567–6572 (2009). Crompton, R. H. et al. The evolution of the human foot: evidence from Pliocene hominins. J. Anat. 212 , 443–462 (2008). Bennett, M. R. et al. Early hominin foot morphology based on 3.66-million-year-old footprint evidence from Laetoli. Tanzan. Nat. 478 , 519–523 (2011). Domínguez-Rodrigo, M. et al. Fossil footprints from Laetoli (Tanzania) provide evidence for marked variation in early hominins. Sci. Adv. 6 , eaay3902 (2020). Webb, D. & Kuhn, B. The Laetoli footprints: functional and evolutionary significance. J. Hum. Evol. 15 , 437–447 (1986). Day, M. H. & Napier, J. R. Fossil foot bones from Olduvai Gorge. Nature 201 , 969–970 (1964). Zakany, J. & Duboule, D. The role of Hox genes during vertebrate limb development. Curr. Opin. Genet. Dev. 14 , 359–366 (2004). Kmita, M. & Duboule, D. 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Additional Declarations No competing interests reported. Supplementary Files CamastaFirstRayMorphometricData.txt CamastaFirstRayMorphometricData.xlsx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 11 May, 2026 Reviewers agreed at journal 07 May, 2026 Reviewers invited by journal 06 May, 2026 Editor invited by journal 06 Apr, 2026 Editor assigned by journal 02 Apr, 2026 Submission checks completed at journal 02 Apr, 2026 First submitted to journal 31 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9283272","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":638556192,"identity":"c30ee1cf-74c5-4167-afb4-f78377586a40","order_by":0,"name":"Craig A. Camasta","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAsUlEQVRIiWNgGAWjYDADfgYGxgOkaDBgkGxgYCBRi8EBYrXwtx9+uuHHnz/yxscPHzjAUGMTTVCLxJk0s5u9bQaG286kJRxgOJaW20DQRRIMZjd4GwwYtx3IMTjA2HCYGC3s327++WNgv7n/DdFaeMxu87AZJG6QINYWiTM5Zbdl24yTZ9x4lnAggRi/8Lcf33bzzR852/7+5IMPPtTYENaCChJIUz4KRsEoGAWjABcAAO7SQ8zMC/ZCAAAAAElFTkSuQmCC","orcid":"","institution":"Private Practice","correspondingAuthor":true,"prefix":"","firstName":"Craig","middleName":"A.","lastName":"Camasta","suffix":""}],"badges":[],"createdAt":"2026-03-31 17:54:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9283272/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9283272/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":109320872,"identity":"56d0663b-c2ad-4247-a295-fb62b0717eca","added_by":"auto","created_at":"2026-05-15 13:33:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":378909,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eFirst Ray and Second Toe Homology.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e\u003cbr\u003e\nDry-bone comparison of the human first ray and second toe, dorsal-plantar (top) and lateral (bottom) views illustrating serial homology. The proportional equivalence of MT1 / PP1 / DP1 to PP2 / MP2 / DP2 demonstrates that the first ray preserves a canonical three-segment digital architecture, with MT1 morphologically homologous to PP2.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9283272/v1/a8b333fb3d755a832c617349.png"},{"id":109405616,"identity":"286c1ed2-ad14-4bdb-aad6-8885fde1bea6","added_by":"auto","created_at":"2026-05-17 13:19:24","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":231499,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eFirst Ray Macrodactyly and Microdactyly.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e\u003cbr\u003e\nClinical radiographs showing first ray proportional enlargement (macrodactyly, top) and reduction (microdactyly, bottom) in an achondroplastic patient. These extreme variant examples highlight the preservation of phalangeal proportions maintaining distal-proximal segment relationships approximating 0.79, 1.0, 2.1. The consistent scaling of these pathologic variants underscores that MT1 follows the proportional rules of a three-phalanx digit rather than those of a variable metatarsal series.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9283272/v1/27a4f0f79f080eadfd84d8e1.png"},{"id":109405289,"identity":"a3fc4aed-e883-4c5d-a33c-dc58318915fe","added_by":"auto","created_at":"2026-05-17 13:16:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":612574,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9283272/v1/eaf99f81-d16f-4fd7-b175-b0d46b9d2f96.pdf"},{"id":109405575,"identity":"5eaa9422-00fc-49f6-a425-711f97c3d44e","added_by":"auto","created_at":"2026-05-17 13:19:10","extension":"txt","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":3699,"visible":true,"origin":"","legend":"","description":"","filename":"CamastaFirstRayMorphometricData.txt","url":"https://assets-eu.researchsquare.com/files/rs-9283272/v1/6d125c066e2c30ca478681e9.txt"},{"id":109405438,"identity":"0a05eb58-d748-4adf-8321-7151e0414c69","added_by":"auto","created_at":"2026-05-17 13:18:05","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":24906,"visible":true,"origin":"","legend":"","description":"","filename":"CamastaFirstRayMorphometricData.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-9283272/v1/a3246fa6fb0303ce2854d0f9.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"A phalanx in the metatarsus: the human first ray preserves ancestral digital identity","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eClassical anatomists recognized that the first ray\u0026mdash;the hallux with its supporting bones (MT1, proximal and distal phalanges)\u0026mdash;differs markedly from the lesser rays (MT2\u0026ndash;5 with their proximal, middle, and distal phalanges) in shape and mobility. Aristotle\u0026sup1; and Galen\u0026sup2; described the big toe as a distinct \u0026ldquo;thumb of the foot,\u0026rdquo; and Vesalius\u0026sup3; noted that it possessed only two phalanges, unlike the remaining digits that have three.\u003c/p\u003e \u003cp\u003eFrom a functional and evolutionary point of view, the human foot has been characterized by derived features associated with habitual bipedalism, including (i) loss of plantar grip; (ii) loss of prehensile capacity; (iii) loss of mobility of the first ray, which has become permanently adducted against the second; and (iv) medial translation of the functional axis from the primitive third-ray location (mesaxony) to a second-ray configuration (entaxony).⁴ Morton synthesized these observations into a functional framework in which the first ray is a specialized propulsive lever within the medial column,⁵ while Wood Jones similarly emphasized the transformation of the hallux from a prehensile structure into a rigid, adducted element aligned with the medial longitudinal arch.⁶ Recent morphometric, embryological, and genetic evidence challenges the assumption that MT1 is merely a modified metapodial, suggesting instead that the first ray may represent a phalanx-derived digit incorporated proximally within the tarsometatarsal column. The present study integrates osteometric, vascular, ligamentous, developmental, and genetic evidence to test the hypothesis that the first metatarsal is not a metapodial by identity, but a proximally displaced proximal phalanx.\u003c/p\u003e"},{"header":"OSTEOLOGY AND MORPHOMETRICS","content":"\u003cp\u003eTo evaluate the structural identity (rather than positional classification) of the human first ray, osteometric analyses were performed on 100 adult dry-bone foot specimens. The first ray was analyzed as a three-segment unit (MT1\u0026thinsp;+\u0026thinsp;PP1\u0026thinsp;+\u0026thinsp;DP1) and compared with the second toe (PP2\u0026thinsp;+\u0026thinsp;MP2\u0026thinsp;+\u0026thinsp;DP2), which represents a canonical three-phalangeal digital series. This analysis tests whether the proximal element of the first ray conforms to a metapodial or a proximal-phalangeal morphometric program. ⁷\u003c/p\u003e \u003cp\u003eMaximum proximodistal length and mediolateral diaphyseal width were measured for metatarsals I\u0026ndash;V and for the proximal phalanx of digit II (PP2). Length-to-width (L/W) ratios were calculated to characterize relative scale independent of size (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLength-to-width (L/W) ratios of metatarsals I\u0026ndash;V and proximal phalanx II (PP2)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElement\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMean L/W\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSE\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e95% CI\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ep (vs MT1)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCV (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMT1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.83ᵃ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.048\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.73\u0026ndash;4.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e9.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMT2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.00ᶜ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e9.77\u0026ndash;10.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e11.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMT3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.34ᵈ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.104\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10.13\u0026ndash;10.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e10.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMT4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.02ᶜ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.101\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e9.82\u0026ndash;10.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e10.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMT5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.65ᵇ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.128\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e8.39\u0026ndash;8.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e14.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePP2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.68ᵃ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.078\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.53\u0026ndash;4.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e10.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eMean length-to-width (L/W) ratios are shown for metatarsals I\u0026ndash;V (MT1\u0026ndash;MT5) and the proximal phalanx of digit II (PP2) across 100 adult human feet. Values are reported as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard error (SE), with 95% confidence intervals. Group differences were assessed using a randomized-block ANOVA (feet as blocks; skeletal elements as treatments), followed by Tukey\u0026rsquo;s honestly significant difference (HSD) post-hoc test. Elements sharing the same superscript letter do not differ significantly at α\u0026thinsp;=\u0026thinsp;0.05. MT1 and PP2 form a statistically indistinguishable group, distinct from metatarsals II\u0026ndash;V, supporting serial homology of the first ray proximal element with a proximal phalanx.\u003c/em\u003e \u003c/p\u003e \u003cp\u003eWhole-ray architecture was assessed by normalizing segment lengths to the middle segment (PP1 for the first ray; MP2 for the second toe), allowing direct comparison of internal digital proportions (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Scaling relationships between length and width were examined to determine whether MT1 aligns with metapodial or phalangeal morphospace (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), and normalized segmental proportions are summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e "},{"header":"RESULTS","content":"\u003cp\u003eMorphometric analyses demonstrate that the first metatarsal occupies a morphospace distinct from the metatarsal series and congruent with proximal phalanges. The mean length-to-width (L/W) ratio of MT1 (4.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.48) was less than half that of metatarsals II\u0026ndash;V (8.65\u0026ndash;10.34; all p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001), while MT1 and the proximal phalanx of digit II (PP2) did not differ significantly (p\u0026thinsp;=\u0026thinsp;0.73; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePatterns of variability further distinguish MT1 from the metatarsal series. The length/width ratios for MT1 closely matched that of PP2 and these were substantially lower than those of metatarsals II\u0026ndash;V. This indicates shared developmental constraint within a phalangeal module rather than the broader variability characteristic of metapodial elements. Coefficient of variance being relatively equal for all elements implies a common developmental constraint.\u003c/p\u003e \u003cp\u003eWhole-ray proportional analysis revealed that the first ray preserves a canonical three-segment digital architecture. After normalization to the middle segment, proximal and distal segment ratios of the first ray clustered near 2.1 and 0.79, respectively, and were indistinguishable from those of the second toe (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). These proportional relationships were maintained across the full range of absolute sizes observed, demonstrating conservation of internal digital scaling despite variation in overall ray length.\u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThese morphometric data demonstrate that the first metatarsal does conform to a phalanx-like element incorporated proximally within the first ray. MT1 matches proximal phalanges in absolute proportions, variance, and length\u0026ndash;width scaling, while remaining clearly distinct from the canonical metatarsal program governing rays II\u0026ndash;IV.\u003c/p\u003e \u003cp\u003eWhole-ray proportional analysis is particularly revealing. Despite its proximal position, the first ray preserves a three-segment digital architecture indistinguishable from that of the second toe. This pattern is incompatible with interpretations of MT1 as a hypertrophied or remodeled metapodial, but follows directly if MT1 represents a proximally displaced proximal phalanx retaining its ancestral scaling relationships.\u003c/p\u003e \u003cp\u003eTogether with its low L/W ratio, and preservation of canonical three-segment digital proportions, these findings indicate that MT1 is a morphometrically constrained, phalanx-like element rather than a modified metapodial. As shown in the sections that follow, this morphometric signal is not coincidental but recurs across independent anatomical, developmental, vascular, and evolutionary observations that together bear on the identity of the first ray.\u003c/p\u003e\n\u003ch3\u003eLIGAMENTOUS AND JOINT ARCHITECTURE\u003c/h3\u003e\n\u003cp\u003eThe first tarsometatarsal joint (TMT1) is anatomically and synovially isolated from the lesser tarsometatarsal complex. Unlike TMT2\u0026ndash;TMT5, which are united within a continuous synovial compartment\u0026mdash;the classical \u0026ldquo;great tarsal synovial cavity\u0026rdquo;\u0026mdash;TMT1 possesses a discrete, independent joint capsule with no synovial communication to adjacent tarsometatarsal joints. ⁸\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1;⁰\u003c/p\u003e \u003cp\u003eMetatarsals II\u0026ndash;V are bound together by confluent dorsal, plantar, and interosseous ligaments, forming a rigid intermetapodial unit reinforced by the Lisfranc ligament, which spans from the medial cuneiform to the base of MT2. TMT1 is excluded from this complex entirely: it lacks interosseous connections to the lesser metatarsals, is bypassed by the Lisfranc ligament, and articulates through an autonomous capsular system.\u003c/p\u003e \u003cp\u003eThis synovial segregation is not a minor variation in mobility but a categorical distinction in joint organization. Whereas TMT2\u0026ndash;TMT5 function as a unified metapodial block, TMT1 behaves as an independent articulation, structurally and developmentally analogous to a metatarsophalangeal joint rather than a true intermetapodial junction. The isolated capsule and absence of shared synovial space reinforce the interpretation that the first ray is excluded from the Lisfranc complex, consistent with preserved phalangeal identity rather than a modified metapodial element.\u003c/p\u003e\n\u003ch3\u003eVASCULAR AND PHYSEAL PARALLELS\u003c/h3\u003e\n\u003cp\u003eMT1 exhibits a vascular and physeal pattern parallel to the phalanges rather than the metatarsals. Its nutrient artery enters proximally on the plantar base, a configuration shared by proximal phalanges but not by metatarsals II\u0026ndash;V, whose nutrient foramina are typically mid-diaphyseal.\u0026sup1;\u0026sup1;\u003csup\u003e,\u003c/sup\u003e\u0026sup1;\u0026sup2; Likewise, MT1 possesses a proximal epiphysis, matching phalangeal polarity rather than the distal epiphyseal orientation of the lesser metatarsals.\u0026sup1;\u0026sup3; Together, these features indicate that MT1 follows a phalangeal\u0026mdash;not metapodial\u0026mdash;developmental program.\u003c/p\u003e\n\u003ch3\u003eEMBRYONIC FOOT DEVELOPMENT AND THE OS INTERMETATARSEUM\u003c/h3\u003e\n\u003cp\u003eEmbryonic skeletal elements of the human foot develop in a skewed, non-metameric order of chondrification that places the first ray out of sequence with the lesser metatarsals. Metatarsals II\u0026ndash;V chondrify first, followed by delayed appearance of MT1 and then the proximal phalanges of digits II\u0026ndash;V. Within the hallux, PP1 forms just prior to the middle phalanges of digits II\u0026ndash;V, while DP1 develops in parallel with DP2\u0026ndash;DP5.\u0026sup1;⁴\u003csup\u003e,\u003c/sup\u003e\u0026sup1;⁵ This sequence aligns the first ray developmentally with the digital program of rays II\u0026ndash;V rather than with the metatarsal cohort.\u003c/p\u003e \u003cp\u003eDuring this same interval, a constant transient primordium appears between the bases of MT1 and MT2 within the embryonic metatarsus, corresponding to the os intermetatarseum (OI).\u0026sup1;⁵\u003csup\u003e,\u003c/sup\u003e\u0026sup1;⁶ At the 25 mm crown\u0026ndash;rump length this primordium may chondrify, but it quickly dedifferentiates and rapidly disappears after reaching the 30 mm crown\u0026ndash;rump length.\u0026sup1;⁵ At maximal development, it attains a quarter of the length of the first intermetatarsal space and \u0026ldquo;bears the character of a structure recapitulated in the course of ontogenesis.\u0026rdquo;\u0026sup1;⁵ In adults, the OI may persist as an accessory ossicle or fuse variably to the base of either MT1 or MT2.\u0026sup1;⁷\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1;⁹\u003c/p\u003e \u003cp\u003eThe identity of this element has been debated for more than a century. Gruber interpreted it as \u0026ldquo;split-off material of an originally bulkier and longer tarsale distale I,\u0026rdquo;\u0026sup1;⁶ while Pfitzner proposed that it represented a persisting ancestral metatarsal or tarsal element.\u0026sup2;⁰ Čih\u0026aacute;k rejected this interpretation, noting that \u0026ldquo;in paleontology there is known neither a case of six elements in the distal tarsal row nor an increased number in Theromorphs,\u0026rdquo; which \u0026ldquo;are all five-toed.\u0026rdquo;\u0026sup1;⁵ Slab\u0026yacute; later emphasized that \u0026ldquo;phylogenetic evidence can scarcely be found for the theory which considers this bone a persisting ancestral metatarsal element.\u0026rdquo;\u0026sup2;\u0026sup1;\u003c/p\u003e \u003cp\u003eArising at the 16\u0026ndash;30 mm crown\u0026ndash;rump length, the OI appears before the delayed chondrification of MT1 and in parallel with MT2\u0026ndash;MT5, placing this vestigial element within the ancestral metapodial domain and underscoring MT1\u0026rsquo;s displacement from the typical metatarsal program toward a phalangeal identity.\u0026sup1;⁰\u003csup\u003e,\u003c/sup\u003e\u0026sup1;⁴\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1;⁶\u003c/p\u003e \u003cp\u003ePostnatal skeletal maturation mirrors this embryonic distinction. The proximal physis of MT1 ossifies completely at approximately 15\u0026ndash;17 years, coincident with closure of the phalangeal physes, whereas the distal growth plates of metatarsals II\u0026ndash;V typically remain open until late adolescence (~\u0026thinsp;17\u0026ndash;20 years). \u0026sup1;⁰\u003csup\u003e,\u003c/sup\u003e\u0026sup1;\u0026sup3; The earlier cessation of growth in MT1 thus conforms to a phalangeal rather than metatarsal maturation schedule.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eHAND HOMOLOGY\u003c/h2\u003e \u003cp\u003eIndependent evidence from the human hand supports the interpretation that the first ray preserves phalangeal identity through proximal displacement. Radiographic and developmental analyses have demonstrated that the first metacarpal (MC1) shares proportional geometry, ossification behavior, and growth dynamics with the proximal phalanges of digits II\u0026ndash;V rather than with the remaining metacarpals. Pazzaglia and colleagues concluded that the missing thumb segment is best accounted for by phalangeal homology of MC1 using proportional length assessment, ossification-center distribution, and differential growth rates,\u0026sup2;\u0026sup2; and Valenzuela and colleagues similarly identified MC1 as homologous to the proximal phalanx based on relative length relationships and delayed maturation of the preaxial digit.\u0026sup2;\u0026sup3;\u003c/p\u003e \u003cp\u003eFurther support comes from analyses of rare five-fingered hand (5-FH) phenotypes in humans, a dominantly inherited condition linked to chromosome 7q36.\u0026sup2;⁴ In these individuals, Bondioni et al. demonstrated that MC1 exhibits postnatal growth asymmetry and proportional scaling indistinguishable from proximal phalanges and distinct from metacarpals II\u0026ndash;V, supporting phalangeal homology of the thumb\u0026rsquo;s proximal element.\u0026sup2;⁵ Developmental studies of the growing hand likewise show delayed ossification and maturation of the preaxial (radial/anterior) digit relative to the postaxial digits, both prenatally and postnatally, mirroring developmental polarity patterns observed in the foot.\u0026sup2;\u0026sup3;\u003c/p\u003e \u003cp\u003eThe concordant pattern in hand and foot reinforces the interpretation that the first ray retains intrinsic phalangeal identity despite occupying a proximal position.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eFUNCTIONAL AND CLINICAL IMPLICATIONS\u003c/h3\u003e\n\u003cp\u003eWhy is MT1 so thick and broad compared with metatarsals II\u0026ndash;V? Multiple pedobarographic studies demonstrate that MT1 does not bear the greatest plantar pressure during gait,\u0026sup2;⁶\u003csup\u003e,\u003c/sup\u003e\u0026sup2;⁷ with peak pressures instead concentrated beneath the hallux and central forefoot rather than the first metatarsal head.\u0026sup2;⁸ Reliability of the measurement for plantar pressure parameters is high.\u0026sup2;⁹ The transverse robustness of MT1 therefore does not reflect increased peak load bearing, but structural resistance and the mechanical demands of the sesamoid\u0026ndash;pulley apparatus. Its shape is optimized for leverage during terminal stance, consistent with reinforcement of a proximal-phalangeal element rather than hypertrophy of a metapodial.\u0026sup3;⁰\u003c/p\u003e \u003cp\u003eThe metatarsal parabola (MT2\u0026thinsp;\u0026gt;\u0026thinsp;MT3\u0026thinsp;\u0026gt;\u0026thinsp;MT4\u0026thinsp;\u0026gt;\u0026thinsp;MT5) is highly conserved across individuals. By contrast, MT1 exhibits greater variability in projection length relative to MT2 across both osteometric and radiographic analyses.\u0026sup3;\u0026sup1;\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup3;⁴ Some feet display a relatively longer MT1, whereas others exhibit a shorter one; nevertheless, this variability does not alter the internal proportional scaling of the first ray relative to the second toe. Pathologic extremes of first-ray macrodactyly and microdactyly demonstrate preservation of proximal-to-distal segment ratios despite marked size differences (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \n\u003ch3\u003eCOMPARATIVE AND EVOLUTIONARY CONTEXT\u003c/h3\u003e\n\u003cp\u003eComparative and fossil evidence show that the two-phalangeal condition of digit I is among the most conserved features of the tetrapod limb. Early tetrapods rapidly stabilized a pentadactyl pattern in which digit I possessed fewer phalanges than the central digits. By the time of early amniotes, a digital formula approximating 2-3-4-5-3 was established, and subsequent synapsid and mammalian evolution consolidated this pattern into the canonical mammalian formula 2-3-3-3-3.\u003c/p\u003e \u003cp\u003eThis extraordinary persistence indicates that digit I occupies a uniquely constrained developmental domain within the autopod. Whereas digits II\u0026ndash;V underwent progressive reduction and reorganization during amniote diversification, digit I consistently retained a short, two-phalangeal architecture. Such long-term stability favors preservation of distal digital identity rather than repeated modification of a metapodial element.\u003c/p\u003e \u003cp\u003eIn primates, the first ray maintained pronounced medial mobility and a grasping role. Fossil hominins document its subsequent transformation into a load-bearing component of the human foot. In \u003cem\u003eArdipithecus ramidus\u003c/em\u003e (~\u0026thinsp;4.4 Ma), the first ray remained abducted and opposable, yet already exhibited transverse robustness within the modern human range, indicating structural reinforcement prior to the evolution of rigid longitudinal and transverse arches.\u0026sup3;⁵ Through \u003cem\u003eAustralopithecus afarensis\u003c/em\u003e and early \u003cem\u003eHomo\u003c/em\u003e, the first ray progressively adducted toward the second ray, integrating into the developing medial column of the foot.\u0026sup3;⁶\u003csup\u003e\u0026minus;\u003c/sup\u003e⁴\u0026sup3;\u003c/p\u003e \u003cp\u003eThe modern human foot represents reorganization rather than a departure from ancestral patterning. Despite profound functional change\u0026mdash;from grasping to propulsion\u0026mdash;the intrinsic digital formula of the first ray was preserved. Evolutionary innovation proceeded through spatial redeployment of conserved developmental elements, not through reversal of polarity or loss of digital identity (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eContinuity of the two-phalangeal first digit across vertebrate evolution.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTaxon / Group\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eApprox. Age (Ma)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePhalangeal Formula (I\u0026ndash;V)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eKey Features\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePolydactyl stem-tetrapods\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;365\u0026ndash;360\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;5 digits\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTransitional limbs with variable digit counts\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFirst pentadactyl tetrapods\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;360\u0026ndash;350\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-4-5-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFixed five rays; digit I already two phalanges\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBasal amniotes\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;320\u0026ndash;310\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-4-5-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eConserved two-phalangeal digit I\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEarly synapsids\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;305\u0026ndash;275\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-4-4-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDigit I stable; central digits reduced\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBasal mammals\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;200\u0026ndash;160\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-3-3-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMammalian formula fixed\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePrimates\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;66\u0026ndash;50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-3-3-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eOpposable hallux; grasping feet\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eArdipithecus ramidus\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;4.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-3-3-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAbducted hallux; elongate and robust first ray\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eAustralopithecus afarensis\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;3.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-3-3-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eInline hallux; incipient arch\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eHomo sapiens\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2-3-3-3-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFull arches; rigid first ray\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eChronological summary of phalangeal formulas (digits I\u0026ndash;V) from early tetrapods to Homo sapiens, illustrating the long-term conservation of the two-phalangeal condition of digit I. Progressive reduction and consolidation of central digits produced the canonical mammalian pattern 2-3-3-3-3, retained through primates and hominins and foundational to the preserved-polarity model of the human first ray.\u003c/em\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eHOX GENE REGULATION AND PRESERVED POLARITY\u003c/h2\u003e \u003cp\u003ePatterning of the vertebrate limb is governed by the Hox gene family, a conserved group of transcription factors that establish positional identity along the proximal\u0026ndash;distal axis of the appendicular skeleton. This axis is classically divided into the stylopod (proximal segment), zeugopod (intermediate segment), and autopod (distal segment comprising the hands and feet). Within the autopod, distinct combinations of Hox gene expression specify metapodial versus phalangeal identity and regulate digital segmentation. ⁴⁴\u003csup\u003e\u0026minus;\u003c/sup\u003e⁵⁰\u003c/p\u003e \u003cp\u003eDigit I occupies a uniquely truncated developmental domain within this framework. Unlike digits II\u0026ndash;V, which express overlapping domains of Hoxd11, Hoxd12, and Hoxd13, digit I is specified almost exclusively by Hoxd13, with little or no contribution from the more proximal paralogues. This restricted expression domain establishes intrinsic phalangeal identity and limits segmentation, accounting for the conserved two-phalangeal structure of digit I across tetrapods.⁴⁴\u003csup\u003e\u0026minus;\u003c/sup\u003e⁴⁸\u003c/p\u003e \u003cp\u003eClassical reverse-polarity proponents interpret the first metatarsal as a true metapodial that secondarily acquired phalangeal characteristics through inversion of developmental polarity.⁵\u0026sup1; Under this model, MT1 is presumed to retain metapodial identity despite its phalange-like morphology, with distal regulatory programs imposed upon an originally proximal skeletal element.\u003c/p\u003e \u003cp\u003eIn the human first ray, however, the distal identity program is not reversed but spatially displaced. MT1 resides within a Hoxd13-dominant regulatory field rather than within the mixed Hox domains characteristic of metatarsals II\u0026ndash;V. This placement is consistent with MT1\u0026rsquo;s phalangeal morphometrics, proximal vascular entry, proximal physeal polarity, delayed chondrification, and ligamentous isolation. Polarity is therefore preserved rather than inverted: the ancestral first metapodial program was developmentally suppressed\u0026mdash;its vestige appearing as the os intermetatarseum\u0026mdash;while phalangeal identity expanded proximally into the tarsometatarsal column.\u003c/p\u003e \u003cp\u003eExperimental manipulation of Hox13 paralogues supports this interpretation. Loss of Hox13 function disrupts phalangeal segmentation, whereas ectopic activation of Hoxd13 transforms metapodial condensations into phalangeal-like elements. Conversely, deletion of distal regulatory enhancers collapses multiple phalanges into a single condensation. Together, these findings demonstrate that Hox13 paralogues act as terminal selector genes for phalangeal identity. In the first ray, this identity module has been redeployed proximally without alteration of its intrinsic developmental logic.\u003c/p\u003e \u003cp\u003eThe preserved-polarity model thus resolves the apparent paradox of the human first ray. Rather than representing a metapodial that secondarily acquired phalangeal features, MT1 is a phalangeal element by identity, incorporated proximally through spatial reorganization of conserved developmental programs. This interpretation integrates genetic regulation with embryologic timing, comparative anatomy, and morphometric constraint, and provides a mechanistic foundation for understanding the evolutionary transformation of the human foot.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eREASSESSMENT OF THE REVERSE-POLARITY MODEL\u003c/h2\u003e \u003cp\u003eReno and colleagues proposed that the first metatarsal represents a true metapodial that secondarily acquired phalangeal features through reversal of developmental polarity. ⁵\u0026sup1; Under this reverse-polarity model, MT1 is interpreted as homologous with metatarsals II\u0026ndash;V despite its phalange-like proportions, proximal epiphysis, and atypical vascular and ligamentous anatomy. The defining criterion for metapodial identity in this framework is positional: articulation with a distal tarsal is taken to supersede developmental, morphometric, and genetic evidence.\u003c/p\u003e \u003cp\u003eWhen evaluated using the evidence presented here, this model is not parsimoniously supported. First, positional articulation alone does not define serial homology. The os intermetatarseum occupies the predicted locus of the ancestral first metapodial between the medial cuneiform and MT2, indicating that the original metapodial condensation was developmentally suppressed rather than transformed. Recognition of this element resolves the positional paradox without invoking polarity reversal.\u003c/p\u003e \u003cp\u003eSecond, the anatomical features cited as evidence for polarity reversal form a coherent phalangeal signature rather than a labile metapodial variant. MT1 consistently exhibits phalange-like morphometrics, low proportional variance, proximal nutrient-artery entry, proximal physeal polarity, delayed chondrification, and isolation from the Lisfranc complex. These traits co-occur across specimens and developmental stages and are shared with proximal phalanges but not with metatarsals II\u0026ndash;V. Concidering their concordance as independent reversals of multiple developmental axes is not parsimonious.\u003c/p\u003e \u003cp\u003eThird, the reverse-polarity model lacks a mechanistic genetic foundation. Hox-gene regulation predicts that elements specified within a Hoxd13-dominant domain will express phalangeal identity, irrespective of their spatial position. MT1 resides within this distal regulatory field, whereas metatarsals II\u0026ndash;V do not. Experimental manipulation of Hox13 paralogues demonstrates that phalangeal identity cannot be imposed upon a metapodial framework without altering its developmental program. The observed phenotype of MT1 therefore reflects preserved distal identity rather than inverted polarity.\u003c/p\u003e \u003cp\u003eFinally, the reverse-polarity hypothesis implies repeated, independent loss of a true first metapodial across tetrapod evolution. In contrast, the preserved-polarity model requires only a single ancestral suppression event, with the os intermetatarseum persisting as a vestigial remnant. This explanation is more parsimonious and aligns with the long-term conservation of first ray phalangeal identity documented across more than 350\u0026nbsp;million years of vertebrate evolution.\u003c/p\u003e \u003cp\u003eTaken together, the reverse-polarity interpretation fails to accommodate the combined osteometric, developmental, vascular, ligamentous, genetic, and comparative evolutionary evidence. The preserved-polarity model accounts for all observed features within a single, mechanistically grounded framework: MT1 is a proximal phalanx by identity, incorporated proximally through spatial reorganization of conserved developmental programs rather than through reversal of limb polarity.\u003c/p\u003e \u003cp\u003eReno et al. correctly emphasize that TMT1 is ligamentously and capsularly segregated from the Lisfranc complex, but their proposed mechanism is inverted. They argue that a proximal growth plate \u0026ldquo;precluded\u0026rdquo; intermetatarsal union in the first ray, with compensatory joint-shape changes. Under the preserved-polarity model, however, the absence of intermetatarsal ligaments is not a deficiency produced by an unusual physis; it is the expected anatomy of a non-metapodial element occupying the first-ray position. In the lateral rays, metapodial identity is expressed as a shared synovial\u0026ndash;ligamentous complex (the Lisfranc unit) that couples MT2\u0026ndash;MT5 into a stabilized platform with minimal triplanar excursion. By contrast, TMT1 behaves as an isolated, triplanar articulation precisely because its proximal element is phalangeal by identity: it lacks the intermetapodial \u0026ldquo;zippering\u0026rdquo; architecture that defines the metatarsal series. This interpretation simultaneously accounts for (i) the absence of Lisfranc linkage, (ii) proximal physeal polarity, (iii) proximal nutrient-artery entry, (iv) delayed chondrification sequence, and (v) phalangeal scaling, without invoking a compensatory narrative or independent polarity reversals.\u003c/p\u003e \u003cp\u003eTaken together, the ligamentous, vascular, developmental, morphometric, and genetic evidence converges on element identity rather than positional reinterpretation.\u003c/p\u003e \u003cp\u003eThese findings fulfill Remane\u0026rsquo;s criteria for homology\u0026mdash;special quality, topological relations, and continuity through intermediates\u0026mdash;demonstrating that MT1 reproduces phalangeal geometry and developmental polarity while maintaining continuity through the os intermetatarseum and fossil intermediates. \u003csup\u003e52\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eDISCUSSION AND SYNTHESIS\u003c/h2\u003e \u003cp\u003eThe human first ray exemplifies how profound functional transformation can arise through developmental conservation rather than developmental reversal. In ancestral primates, the hallux was shortened, opposable, and constrained within a phalangeal identity suited for grasping. With the advent of habitual bipedalism, the first ray was progressively lengthened, adducted, and reinforced to participate in the medial longitudinal and transverse arches of the foot. Fossil intermediates demonstrate that transverse robustness of the first ray preceded full arch rigidity, indicating structural strengthening before the complete loss of prehensility.\u003c/p\u003e \u003cp\u003eCrucially, this transformation did not require reversal of developmental polarity. Instead, a phalanx-derived element was incorporated proximally into the tarsometatarsal column, preserving intrinsic digital identity while accommodating new mechanical demands. The os intermetatarseum represents the vestigial remnant of a suppressed ancestral metapodial, revealing that the tetrapod limb is fundamentally pentadactyl and triphalangeal, yet quadrametapodial in its developmental architecture.\u003c/p\u003e \u003cp\u003eThe modern human first ray thus represents a robust, phalanx-derived element whose proportions and scaling preserve those of a typical three-segment digit, repurposed for upright locomotion. Bipedal propulsion was achieved not by abandoning ancestral patterning, but by spatial reorganization of conserved developmental modules. In this light, the apparent uniqueness of the human first ray reflects not evolutionary novelty, but the long-term stability and flexibility of a deeply conserved developmental framework.\u003c/p\u003e \u003cp\u003eIn this context, the first ray illustrates a broader evolutionary tendency toward the reuse and reorganization of established anatomical elements rather than the generation of novel ones.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eMethods – Statistical Analysis\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;Morphometric analyses were conducted using Python (v3.11) with the SciPy and StatsModels libraries. For each specimen, proximodistal length and mediolateral diaphyseal width were measured for metatarsals I–V (MT1–MT5) and for the proximal phalanx of digit II (PP2). Length-to-width (L/W) ratios were calculated to quantify relative robustness independent of absolute size.\u003c/p\u003e\n\u003cp\u003eTo account for repeated measures within individual feet, group-level differences in L/W ratios were evaluated using a randomized-block analysis of variance (ANOVA), with feet treated as blocks and skeletal elements treated as fixed effects. Post-hoc pairwise comparisons were performed using Tukey’s Honestly Significant Difference (HSD) test. Group means are reported with standard errors (SE) and 95% confidence intervals. CV = (s/ȳ) x 100. All methods were carried out in accordance with relevant guidelines and regulations. \u0026nbsp;All raw data and variable definitions are publicly available, enabling independent verification and replication of all reported analyses.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll raw morphometric data generated and analyzed during this study are publicly available in the Dryad Digital Repository (https://doi.org/10.5061/dryad.9s4mw6mxp).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo custom code was used in this study. All analyses were performed using standard statistical methods as described in the Methods section.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll methods were carried out in accordance with relevant guidelines and regulations. The material analyzed in this study consisted of de-identified adult human dry-bone foot specimens from the Department of Anatomy at the Pennsylvania College of Podiatric Medicine (now Temple University School of Podiatric Medicine), examined as part of an institutional anatomical teaching and research collection. No living subjects, patient data, or identifiable human materials were involved. Institutional ethical approval and the requirement for informed consent were waived by the Institutional Review Board of Temple University School of Podiatric Medicine.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe author declares no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere is no funding for this project from any outside sources.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor’s Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCraig A. Camasta, DPM, FACFAS\u003c/p\u003e\n\u003cp\u003ePrivate Practice, Atlanta Georgia USA 30062\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding Author\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCraig A. Camasta, DPM, FACFAS\u003c/p\u003e\n\u003cp\u003eEmail: [email protected]\u003c/p\u003e\n\u003cp\u003ePhone: (1) 678-549-8078\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAristotle Parts of Animals, Book IV.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGalen De Usu Partium Corporis Humani (On the Usefulness of the Parts of the Body).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVesalius, A. \u003cem\u003eDe Humani Corporis Fabrica Libri Septem\u003c/em\u003e (Oporinus, 1543).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLe Minor, J. M., Mousson, J. F., de Mathelin, P. \u0026amp; Bierry, G. Non-metric variation of the middle phalanges of the human toes (II\u0026ndash;V): long/short types and their evolutionary significance. \u003cem\u003eJ. 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Evol.\u003c/em\u003e \u003cb\u003e320\u003c/b\u003e, 1\u0026ndash;15 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRemane, A. \u003cem\u003eDie Grundlagen des nat\u0026uuml;rlichen Systems, der vergleichenden Anatomie und der Phylogenetik\u003c/em\u003e (Akademische Verlagsgesellschaft, 1952).\u003c/span\u003e\u003c/li\u003e \u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-9283272/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9283272/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe human first metatarsal (MT1) has long been regarded as a true metapodial uniquely adapted for bipedal propulsion. Yet developmental, vascular, and morphometric evidence increasingly contradicts this view. Across 100 adult human foot specimens, MT1 shares its proportions, nutrient-artery entry, and physeal orientation with proximal phalanges rather than with metatarsals II\u0026ndash;V. Developmentally, MT1 arises after the other metatarsals but before the phalanges, defining a proximal-phalangeal timing sequence. Its joint and ligamentous architecture reveals isolation from the Lisfranc complex, while its gene-expression domain lies within the distal Hox field characterized by Hoxd13-dominant regulation. Collectively, these features identify MT1 as a proximally displaced phalanx, with the os intermetatarseum representing a suppressed ancestral metapodial. This reinterpretation reframes the human foot as a pentadactyl yet quadrametapodial limb, integrating anatomy, development, and evolution within a preserved-polarity model grounded in fossil and comparative anatomical evidence.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"A phalanx in the metatarsus: the human first ray preserves ancestral digital identity","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-15 13:33:01","doi":"10.21203/rs.3.rs-9283272/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"296816616878132761647540720391666985724","date":"2026-05-11T21:13:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"71307723654739009209084129879635239454","date":"2026-05-07T13:36:01+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-05-06T15:24:40+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-04-06T14:00:37+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-02T04:02:06+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-02T04:01:05+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-03-31T17:39:58+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6af198d6-bf4a-442b-91f8-63d3ebd15310","owner":[],"postedDate":"May 15th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"296816616878132761647540720391666985724","date":"2026-05-11T21:13:23+00:00","index":80,"fulltext":""},{"type":"reviewerAgreed","content":"71307723654739009209084129879635239454","date":"2026-05-07T13:36:01+00:00","index":79,"fulltext":""},{"type":"reviewersInvited","content":"6","date":"2026-05-06T15:24:40+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":67961233,"name":"Health sciences/Anatomy"},{"id":67961234,"name":"Biological sciences/Developmental biology"},{"id":67961235,"name":"Biological sciences/Evolution"},{"id":67961236,"name":"Biological sciences/Zoology"}],"tags":[],"updatedAt":"2026-05-15T13:33:01+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-15 13:33:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9283272","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9283272","identity":"rs-9283272","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}